When Zero turned a year old, we used up what was left of his puppy food and then put him on the adult formula. We didn't have to "transition" him, as we used the same brand.

Now that we have Marley, we are constantly having to monitor that Zero is eating the adult food and not the puppy food, so he doesn't get fat, and that Marley is eating the puppy food and not the adult food, so he gets the nutrients he needs to grow healthy. Throw in Tia coming home in 4 weeks, and that's 3 dogs to monitor, LOL!